* requires mechanical ventilation. * current participation in any other interventional study. * alanine transaminase/aspartate transaminase levels ≥3 times the upper limit of normal (×uln) or total bilirubin (tbili) ≥2×uln. * lymphocyte count \<500 lymphocytes/microliter (μl) or hemoglobin \<11 grams/deciliter (g/dl). * stage 4 severe chronic kidney disease or requiring dialysis (that is, estimated glomerular filtration rate \<30). * any other condition, that in the opinion of the investigator, may be cause to exclude the participant from the study. * use of steroids (except dexamethasone), sensitive cyp2d6 substrates, cyp2c inducers, il-6 neutralizing antibodies, il-6 receptor inhibitors, or any investigational therapy. * pregnancy or breast feeding. * anticipated transfer to another hospital which is not a study site within 72 hours. * known allergy to ptc299 or excipients.

* requires mechanical ventilation. * current participation in any other interventional study. * alanine transaminase/aspartate transaminase levels ≥3 times the upper limit of normal (×uln) or total bilirubin (tbili) ≥2×uln. * lymphocyte count \<500 lymphocytes/microliter (μl) or hemoglobin \<11 grams/deciliter (g/dl). * stage 4 severe chronic kidney disease or requiring dialysis (that is, estimated glomerular filtration rate \<30). * any other condition, that in the opinion of the investigator, may be cause to exclude the participant from the study. * use of steroids (except dexamethasone), sensitive cyp2d6 substrates, cyp2c inducers, il-6 neutralizing antibodies, il-6 receptor inhibitors, or any investigational therapy. * pregnancy or breast feeding. * anticipated transfer to another hospital which is not a study site within 72 hours. * known allergy to ptc299 or excipients.

- requires mechanical ventilation. - current participation in any other interventional study. - alanine transaminase/aspartate transaminase levels ≥3 times the upper limit of normal (×uln) or total bilirubin (tbili) ≥2×uln. - lymphocyte count <500 lymphocytes/microliter (μl) or hemoglobin <11 grams/deciliter (g/dl). - stage 4 severe chronic kidney disease or requiring dialysis (that is, estimated glomerular filtration rate <30). - any other condition, that in the opinion of the investigator, may be cause to exclude the participant from the study. - use of steroids (except dexamethasone), sensitive cyp2d6 substrates, cyp2c inducers, il-6 neutralizing antibodies, il-6 receptor inhibitors, or any investigational therapy. - pregnancy or breast feeding. - anticipated transfer to another hospital which is not a study site within 72 hours. - known allergy to ptc299 or excipients.

- requires mechanical ventilation. - current participation in any other interventional study. - alanine transaminase/aspartate transaminase levels ≥3 times the upper limit of normal (×uln) or total bilirubin (tbili) ≥2×uln. - lymphocyte count <500 lymphocytes/microliter (μl) or hemoglobin <11 grams/deciliter (g/dl). - stage 4 severe chronic kidney disease or requiring dialysis (that is, estimated glomerular filtration rate <30). - any other condition, that in the opinion of the investigator, may be cause to exclude the participant from the study. - use of steroids (except dexamethasone), sensitive cyp2d6 substrates, cyp2c inducers, il-6 neutralizing antibodies, il-6 receptor inhibitors, or any investigational therapy. - pregnancy or breast feeding. - anticipated transfer to another hospital which is not a study site within 72 hours. - known allergy to ptc299 or excipients.

- requires mechanical ventilation. - current participation in any other interventional study. - alanine transaminase/aspartate transaminase levels ≥3 times the upper limit of normal (×uln) or total bilirubin (tbili) ≥2×uln. - lymphocyte count <800 lymphocytes/microliter (μl) or hemoglobin <11 grams/deciliter (g/dl) irrespective of gender. - stage 4 severe chronic kidney disease or requiring dialysis (that is, estimated glomerular filtration rate <30). - any other condition, that in the opinion of the investigator, may be cause to exclude the participant from the study. - use of steroids (except dexamethasone), sensitive cyp2d6 substrates, cyp2c inducers, il-6 neutralizing antibodies, il-6 receptor inhibitors, or any investigational therapy. - pregnancy or breast feeding. - anticipated transfer to another hospital which is not a study site within 72 hours. - known allergy to ptc299 or excipients.

- requires mechanical ventilation. - current participation in any other interventional study. - alanine transaminase/aspartate transaminase levels ≥3 times the upper limit of normal (×uln) or total bilirubin (tbili) ≥2×uln. - lymphocyte count <800 lymphocytes/microliter (μl) or hemoglobin <11 grams/deciliter (g/dl) irrespective of gender. - stage 4 severe chronic kidney disease or requiring dialysis (that is, estimated glomerular filtration rate <30). - any other condition, that in the opinion of the investigator, may be cause to exclude the participant from the study. - use of steroids (except dexamethasone), sensitive cyp2d6 substrates, cyp2c inducers, il-6 neutralizing antibodies, il-6 receptor inhibitors, or any investigational therapy. - pregnancy or breast feeding. - anticipated transfer to another hospital which is not a study site within 72 hours. - known allergy to ptc299 or excipients.